[T3] Dual Carb Shift Vacuum

Craigs List craigslistcsprings at gmail.com
Wed May 7 07:37:22 PDT 2014


"At first I didn't understand what you meant by "second manifold" but
then I realized that you are trying to make this work with
aftermarket carbs. The OE carb setup came with a balance pipe
connecting the 2 carbs and the AT VM was connected to this balance
pipe, just to the right of center. IIRC, the early pipe was about
1/4" OD while the later one was nearly twice that diameter. It was a
steel pipe, bent to fit around the front (transmission side) of the
engine with rubber connections at each end.

With your setup, you're getting only 1(or 2 unevenly spaced) vacuum
pulses per cycle (2 revolutions) of the engine, depending on how your
vacuum ports are done. This makes your vacuum level even more ragged
and might be causing your problem.

I'd suggest adding a balance pipe between manifolds and then Tee the
VM hose off that pipe. The connections to the manifolds should be
such that you get even vacuum pulses from both cylinders on that
side. The latest OE manifolds have 2 vacuum ports on each manifold.
Those connect together and then connect to the balance pipe."

The only thing that scares me about this is that I had a balance tube
between the two manifolds when I had the 34 ICT's. I had this same issue
with those carbs. and not wanting to shift.

I am more than willing to put a vacuum port on all 4 barrels. I am just not
convinced that it will make that much of a difference. Here is a picture of
the setup I had before.

http://images.thesamba.com/vw/gallery/pix/1127904.jpg

I do know where another set of IDF/DRLA shorty manifolds are. I could get
those, drill and tap all four barrels and do it in such a way that I could
combine the 2 ports on either side before they connect to the balance tube.
You are suggesting a rather large OD hose used as the balance tube?

Since I can find another set of manifolds, I think I will start there. If I
had only found a good deal on a squareback with a manual. So many issues to
tackle with this automatic, aftermarket carbs and soon to be type 4 engine.
:/

Adam


On Wed, May 7, 2014 at 8:21 AM, Jim Adney <jadney at vwtype3.org> wrote:

> On 7 May 2014 at 7:11, Craigs List wrote:
>
> > As most of you know I have been chasing a shifting issue lately. To try
> and
> > get some answers I hooked up a vacuum gauge to my manifold vacuum reading
> > last night. The gauge was all over the place. It was a blur all across
> the
> > face of the gauge. When you would rev the car it would settle a little
> bit
> > but not to the point that I could get a reading. What causes this?
>
> It's normal, and caused by the fact that each time a cylinder sucks
> in air the vacuum increases. At idle, the intakes are rather widely
> separated in time, so the gauge tries to respond to the up/down in
> vacuum as each intake valve opens. The blur you see is the result.
>
> At higher rpm, the needle doesn't have time to respond, so what you
> see is less blur and more of an average reading.
>
> > It was brought to my attention that there might not be enough vacuum at
> > higher RPM's to initiate the shift from 2nd to 3rd. In my mind, I though
> a
> > LACK of vacuum initiated the shift. I could be backwards on this thinking
> > because closing the butterflies is what finally tricks the car into
> > shifting. Closing the butterflies would increase the vacuum.
>
> Your second thought is the correct one. Increasing vacuum causes the
> VM to push the upshift.
>
> > I'm just looking on some pointers as to where to move from here. I was
> > thinking of a few things; vacuum canister between manifold and shift
> > modulator, drilling and tapping the second manifold to try and smooth the
> > signal or blowing the car up......just kidding.
>
> A vacuum cannister would smooth out the vacuum pulses, making your
> gauge reading more steady, but, depending on the sizes of the hoses
> connecting things, it would slow down the vacuum response. I don't
> think you'd like the result.
>
> At first I didn't understand what you meant by "second manifold" but
> then I realized that you are trying to make this work with
> aftermarket carbs. The OE carb setup came with a balance pipe
> connecting the 2 carbs and the AT VM was connected to this balance
> pipe, just to the right of center. IIRC, the early pipe was about
> 1/4" OD while the later one was nearly twice that diameter. It was a
> steel pipe, bent to fit around the front (transmission side) of the
> engine with rubber connections at each end.
>
> With your setup, you're getting only 1(or 2 unevenly spaced) vacuum
> pulses per cycle (2 revolutions) of the engine, depending on how your
> vacuum ports are done. This makes your vacuum level even more ragged
> and might be causing your problem.
>
> I'd suggest adding a balance pipe between manifolds and then Tee the
> VM hose off that pipe. The connections to the manifolds should be
> such that you get even vacuum pulses from both cylinders on that
> side. The latest OE manifolds have 2 vacuum ports on each manifold.
> Those connect together and then connect to the balance pipe.
>
> --
> *******************************
> Jim Adney, jadney at vwtype3.org
> Madison, Wisconsin, USA
> *******************************
>
> _______________________________________________
> VWType3.Org mailing list - type3 at vwtype3.org
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>
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